This application claims the benefit of provisional application 60/367,007, filed Mar. 21, 2002, which is incorporated hereby in its entirety by reference.
1. Field of Invention
The present invention relates generally to space objects (including spacecraft, space vehicles, space satellites and space debris) and more particularly to a method and apparatus for lowering the orbit of a space object with respect to the Earth or some other celestial body.
2. Description of Related Art
Earth Orbital Debris Problem
The Earth has a mature and growing orbit debris and satellite problem. As of September 2001 the US Space Command tracks more than 2741 payloads and 6092 debris objects (greater than 10 cm in size) [MSFC Space Environmental and Effects Program (SEE) http://see.msfc.nasa.gov/]. It is estimated that there are more than 100,000 objects with size in the range 1.5 to 10 cm. All these objects are subjected to further impacts of other debris particles that can cause a cascade of debris once a critical mass of material is in an orbit. Occasional impacts of satellites create many fragments and particles. In addition, all these objects are affected by atomic oxygen erosion, which can cause paint to erode from surfaces increasing the debris environment.
In lieu of a national policy, the National Aeronautics and Space Administration (NASA) has established guidelines [xe2x80x9cGuidelines and Assessment Procedures for Limiting Orbital Debrisxe2x80x9d] for mitigation of orbital debris that include limitations on released objects during normal operations, constraints on the probability of debris-causing explosions and collisions, control requirements on reentry location, and post-mission disposal. Post-mission disposal, practically speaking, calls for de-orbit of satellites at the end of their useful life.
The implication of planned de-orbit has been the carrying of additional propulsion system propellants in order to execute these de-orbit maneuvers at the end of mission. Some satellites have sufficient propulsion capability to lower their orbits or entirely de-orbit at end of life, but this requires additional mass. Unfortunately, the propulsive method of de-orbit requires additional mass be carried at launch that otherwise could have been used for mission equipment, and it also requires a cooperative satellite, that is, one that can receive and act on ground commands or execute long stored sequences. Failures of computers, power systems, or other key systems can make propulsive maneuvers impossible. Unfortunately, Earth orbit space vehicles that cannot afford the propulsive mass have no de-orbit provision whatsoever (e.g. Orbcomm satellites).
Reducing the debris problem is very important to the future use of LEO (low-earth orbit). It is now recognized that the debris problem can be mitigated by the elimination of debris-causing operations and by the removal of objects from orbit after they have achieved their useful life (e.g., old space stations, derelict satellites and used launcher stages).
Space Vehicles And Atmospheric Drag
Natural atmospheric drag operates by the exchange of momentum between the space vehicle and the molecules of the atmosphere. At high altitudes, where the atmosphere exhibits free molecular flow (where a molecule travels further than a characteristic length without colliding with another molecule), the space vehicle impacts molecules of air which either bounce off or stick to the space vehicle. If the molecule sticks, it imparts a momentum change to the space vehicle equal to the molecule""s mass times the relative velocity of the molecule. On the other hand if a molecule bounces off the space vehicle, it imparts up to twice the momentum change. At lower altitudes, both drag and lift forces can be used to change the momentum of the space vehicle.
Space vehicles use entry capsules, heat shields or aeroshells, made of ablative materials or of materials that can withstand high temperature, to protect internal elements from the heating caused by atmospheric drag when reentering the atmosphere. Entry capsules, employed since the late 1950s, use atmospheric drag in order to slow space vehicles in the upper reaches of planetary atmospheres so they can descend to the surface of a planet. Also, concepts exist for ballutes (a word formed by a combination of balloon and parachute), which are robust, high-temperature-capable, inflated envelopes, for increasing the area of a space vehicle to reduce its velocity at a higher than usual altitude as compared to an aeroshell.
Entry Capsules
Entry capsules were used in the re-entry of the Mercury, Gemini and Apollo vehicles, and for planetary entry of the Viking Mars landers, the Pioneer Venus probes, the Galileo Jupiter probe and the Mars Pathfinder lander. These devices can be designed to change the natural entry trajectory of a space vehicle within the atmosphere to target the space vehicle to a particular landing zone as in the case of ballistic missile re-entry vehicles or Mars landing systems [Roy Smith, David Bayard and Ken Mease, xe2x80x9cMars precision landing: an integrated estimation, guidance and control simulation,xe2x80x9d Center for Control Engineering and Computation (CCEC) report 98-0918, University of California, Santa Barbara, 1998]. Entry capsules primarily operate in the planetary Thermiospheres (lower levels) and Mesospheres. Entry capsules require massive thermal protection systems to protect the vehicle from the effects of atmospheric entry heating. The entry capsule mass can often be as much as 25% or more of the mass of the space vehicle itself. Extending the sizes of entry capsules to reduce the ballistic coefficient by one or more orders of magnitude is impractical due to the tremendous mass increase required. Furthermore, entry capsules are not amenable to low-volume stowage and use on uncontrolled space debris.
Aerocapture
Changing a planet-relative orbit from a very high-energy elliptical or a hyperbolic flyby orbit to a low energy elliptical or near circular orbit by means of a single pass to the mesospheric zone of an atmosphere is called aerocapture. Aerocapture has been analyzed extensively in recent years for use in orbit capture in order to reduce propulsion requirements. Aerocapture can be used by space vehicles to go into orbit around distant planets or it can be used at Earth to change the orbit of spent launch vehicle upper stages in order to place them into lower altitude, near circular orbits. Aerocapture systems use thermal protective surfaces oriented toward the atmospheric flow, or ram direction, to quickly decelerate a spacecraft (with a single deep atmospheric pass) from high velocity to orbital velocity. As with entry capsules, aerocapture requires the use of relatively massive aeroshells to protect their enclosed space vehicle from damage by atmospheric heating.
Aerobraking
Aerobraking is the use of atmospheric drag forces during repeated periapsis passes through the very high altitude, lower thermospheric zone of the atmosphere to take energy from an orbit as in circularizing an elliptical orbit. Aerobraking can be performed with unprotected space vehicles provided the altitude of the aerobraking process is high enough, where atmospheric densities are low, to reduce severe heating which could damage a space vehicle. When used, aerobrake devices are relatively large surfaces that can be deployed to increase the number of molecule collisions in order to increase the drag force and accelerate the process of reducing orbit energy. Aerobrakes can also function as protective surfaces to isolate the space vehicle from atmospheric heating during the periapsis passes in the atmosphere. If the cross-section area of a spacecraft can be increased, the aerobraking effect will be effective at higher altitudes, reducing heating of spacecraft surfaces, allowing the series of maneuvers to be completed more quickly and more safely.
Where aerobrake structures have been considered to protect the spacecraft and to provide additional drag, they are typically made of rigid structures with thermal protection materials or lightweight metal films stretched between rigid structural members [Gloyer, P., et al, xe2x80x9cAerobraking to Lower Apogee in Earth Orbit with the Small Payload Orbit Transfer (SPORT(trademark)) Microsatellite Vehicle, xe2x80x9d Paper #SSC01-XI-8, presented at the 15th Annual USU Conference on Small Satellites, 2001]. As lightweight as they can be, they are still heavy as compared to an ultra-thin envelope.
Ballutes
The objective of ballute device concepts is to reduce the velocity of a space vehicle quickly in the upper reaches of a planetary atmosphere (lower Thermosphere and Mesosphere). There are two distinct types of ballute concepts, namely heavy and light. Both device concepts may be deployed prior to entry of a space vehicle, and both types can be mounted to the space vehicle directly or attached to it via a tether. Heavy ballute concepts are very robust and made to withstand high levels of aerodynamic heating and force [W. Arnold and F. Bloetscher, xe2x80x9cAerodynamic deployable decelerator performance evaluation program, phase II,xe2x80x9d Goodyear Aerospace Corp., Air Force Flight Dynamics Laboratory, AFDL-TR-6725, 1966]. Concepts for heavy ballutes have them made of heavy heat resistant materials that can operate at high levels of heating and aerodynamic force. Heavy ballutes can be deployed late in an entry trajectory after the peak of aerodynamic heating and deceleration. The ratio of ballistic coefficient of a vehicle with and without a heavy ballute is usually a factor of 2 to 3.
Concepts for light ballutes, though still robust, operate in the Thermosphere at lower levels of aerodynamic heating and force [Hall, J. L. and Andrew K. Le, xe2x80x9cAerocapture Trajectories for Spacecraft with Large, Towed Ballutes, xe2x80x9d Paper # AAS 01-235, AAS/AIAA Space Flight Mechanics Meeting, Santa Barbara, CA February 2001]. Conceptually, light ballutes are made large enough to reduce the aerodynamic heating to a level that can be radiated by the ballute at an acceptable temperature. Light ballutes alter the entry trajectory, reducing the velocity at low atmospheric density levels. Though light ballutes have also been proposed as simple multi-pass aerobraking devices, they are not capable of withstanding the long-term effects of the space environment and the increasing stagnation pressure with orbit decay. Conceptually, light ballutes are made of a thin film material capable of use as a low-pressure pressure balloon and moderate temperatures on the order of 500xc2x0 C., enclosed in a net of load-bearing tapes of a material capable of a comparable temperature and strength. The ratio of ballistic coefficient of a vehicle with and without a light ballute concept is usually a factor of 50 to 150. Overall, ballutes operate at relatively high temperatures (several hundred degrees rise from ambient) and withstand g-forces several times Earth gravity, thus are relatively thick and heavy requiring 15 to 30% of the total mass of the aerocapture spacecraft.
Space Inflatables
Space inflatable systems have been flown in Earth orbit including the Echo-class balloons, a NASA inflatable antenna flight demonstration, and numerous ballistic missile entry vehicle decoy experiments and demonstrations. Typically, these systems are substantially affected by atmospheric drag.
Balloon Satellites
The NASA Echo I balloon was designed for microwave communications relay experiments. After a number of attempts with various space balloon systems, the Echo I balloon was launched successfully on Aug. 12, 1960 from Wallops Island into an 800-mile altitude orbit. Echo I was a 100-ft diameter Aluminized Mylar spherical envelope that weighed 68 kg including inflation system (Clemmons, Jr., D. L., xe2x80x9cThe Echo I Inflation Systemxe2x80x9d, LaRC, NASA TN D-2194, June 1964 and Wilson, A., A History of Balloon Satellites, Journal of the British Interplanetary Society, V34, p. 10-22, January 1981). Inflation compounds included benzoic acid and anthraquinone, a subliming powder. The Echo I envelope was aluminized (2000-Angstrom layer) to provide a reflective surface for microwave communications relay experiments. This was fortunate for the Echo I experiment because without the metallization, it would have disintegrated within months due to the, then unknown, combined atomic oxygen and UV environment. At the time Echo I flew, the orbital debris environment was nil; most of the holing would have been due to meteoroids. Echo I survived until May 24, 1968 when it finally achieved de-orbit and burned up in the atmosphere.
Inflatable-Deployable Space Structures
Large inflatable-deployable space structures are being considered for a variety of different space applications. NASA has conducted an inflatable antenna experiment, which was deployed from the Space Shuttle in 1996 (R. E. Freeland, et al., xe2x80x9cDevelopment of Flight Hardware for a Large Inflatable-Deployable Antenna Experimentxe2x80x9d, and R. E. Freeland, et al., xe2x80x9cSignificance of the Inflatable Antenna Technologyxe2x80x9d). In some cases heavy envelopes are used that contain self-rigidization materials that are not practical for very large-area inflated envelopes. Furthermore these devices are not deployed to de-orbit other space objects. Indeed, for these space structures it is desirable to achieve as long an orbital life as possible. Space inflatables have no provision for inflation gas maintenance and thus, without some method of rigidization of the envelope, spherical shapes degrade into wrinkled ellipsoids as the long duration effects of meteoroids and space debris penetrate the envelopes and produced leakage holes.
Ballistic Missile Decoys
Another example of a space inflatable device is a decoy for simulation of ballistic missile re-entry. These ballistic missile decoys simulate the size, shape, optical and radar reflections, of missiles, as well as features such as spin, coning, and surface temperature. A number of flight tests have been performed in the past and recently (Cassapakis, C. and M. Thomas, xe2x80x9cInflatable Structures Technology Development Overviewxe2x80x9d, AIAA 95-3738). These space inflatables have no provision for inflation gas maintenance since they are used for only a short duration during sub-orbital flight. In addition, ballistic missile decoy envelopes are relatively heavy due to their simulation requirements. Finally, ballistic missile decoys have no requirement for large cross-section area since their ballistic coefficient is driven by the ballistic missile they are designed to simulate.
Additional Orbit-Lowering Applications
The efficient lowering of space objects from orbit using natural atmospheric drag effects has several applications including the safe removal from orbit, or de-orbit, of large heavy space structures, the efficient de-orbit of spent satellites and orbit debris, the removal of spent upper launch vehicle stages from orbit, the performance enhancement of launch vehicle, and the aero-assisted orbit lowering or aerocapture of planetary satellites.
De-orbit of Large Space Structures
There are several satellites of significant size that represent a future threat to people or property on the ground. Under some operational scenarios, it would be desirable to quickly bring such an object to a low altitude where only a very small delta-V (change in velocity, usually using propellant and a nozzle) is necessary to target re-entry to an unpopulated zone. The natural but accelerated de-orbit of a large derelict space station or orbiting space observatories has been examined. The now reentered Russian space station Mir is a good example of the size of an object that could represent a danger. Such an object first needs to be lowered in altitude to so that a small delta-V will precisely target it to a safe re-entry. For a Mir-sized object at 380-km altitude, orbit lowering would require about 3,000 kg of rocket propellant due to the delta-V required and the large mass of the object to be slowed. At the current cost of launching mass into LEO ($10,000 to 20,000 per kg), this propellant would cost 30-60 million dollars to deliver to orbit. If the mass of a device for orbit lowering were significantly smaller than the mass of rocket fuel for this initial orbit lowering process, it could potentially offer considerable savings.
De-orbit of Spent Satellites and Orbit Debris
The majority of space objects that have de-orbited have done so naturally. Most objects in LEO, satellites and orbital debris, will de-orbit in a period from a few weeks to many years, and most bum up in the atmosphere, so that relatively few space vehicles have been observed and reported as falling to Earth, though even this is a growing concern. Atmospheric drag is particularly effective in de-orbiting low altitude satellites especially in periods of high solar activity when the atmospheric density rises at orbit altitudes. In periods of low solar activity or for higher altitude space vehicles, the natural decay process can take many years, during which time the space vehicle is exposed to orbital debris.
Space Station Trash Removal.
A system for space station trash removal was described in U. S. Pat. No. 5,242,134. As illustrated in FIG. 1 of that patent, a bag member (for storing trash) is attached to compacted balloon element which has enough air left in the interior to inflate the balloon in space and yet permit folding in a compacted condition. The balloon is initially maintained in the compacted condition by retaining means which are released when the bag and balloon are ejected into space. (col. 4, lines 14-42.) Then, for example, when the bag is full, an astronaut in a pressure suit takes the bag to an airlock where the retaining means can be released and the unit is thrown into space. (col. 5, lines 7-20.) Once in a vacuum, the balloon inflates. Due to a large cross-sectional area of the balloon relative to its mass, the combined balloon and bag are slowed by atmospheric drag to a much greater extent than the space station. The balloon and bag lose altitude and reenter the atmosphere, whereby the elements and contents are destroyed by aerodynamic heating. (col. 3, line 65 to col. 4 line 4.) As an alternative to inflation due to air in the folds of the balloon, a subliming agent may be used. (col. 3, lines 57-58.)
This systems lacks a subsystem for controlled inflation and pressure maintenance. Relying on inflation from entrapped gases or a subliming agent may create obstacles to implementation including the availability of airlock, the substantial forces that can result when the balloon is exposed to a vacuum, and pressure loss after deployment due to holing.
Launch Vehicle Removal and Performance Enhancement
Many launch vehicles need to jettison spent rockets and propellant tanks prior to reaching final orbit. Sometimes performance optimization dictates that these stages be placed into independent orbits, especially launches to geosynchronous transfer orbit (GTO). Today some launchers require these stages to carry reserve fuels in order to enable de-orbit, thus preventing them from contributing to the orbital debris problem. Performance of these launchers could be improved if a low-mass device were used instead to de-orbit the stage. Such a device provides options for extending performance of expendable and Space Shuttle launch vehicles by enabling low-cost orbit lowering of stages and external tanks from which propulsive, targeted de-orbit can be achieved thus assisting in space object removal before they can become targets for orbit debris impact.
Conclusion
Thus, there is a need for improved methods and devices for lowering orbits of space objects through atmospheric drag.
In one embodiment of the present invention, an apparatus for lowering an orbit of a space object includes an envelope, an inflation system for inflating the envelope, an inflation control system for controlling the inflation system, and attachment hardware for connecting the apparatus and the space object. Inflating the envelope increases an effective drag area of the envelope for increasing atmospheric drag on the envelope.
Additionally the apparatus may include spatial separation element such as a mast or a tether for providing a spatial separation between the envelope and the space object. Alternatively a spatial deployment-deployment system together with a corresponding control system may be employed for creating a spatial separation between the envelope and the space object. A reefing sleeve may enclose a portion of the envelope so that the envelope is protected before the envelope is inflated and the reefing sleeve is separated from the envelope when the envelope is inflated.
Embodiments of the present invention include corresponding methods. In this way the present invention provides improved apparatuses and methods for lowering the orbits of space objects through atmospheric drag.